The invention relates to an electric proximity sensor, which is sealed tight with respect to pressure fluid.
The preferred embodiment disclosed herein has a casing sleeve, one end of which is closed by a closing member of electrically insulating material, behind which there is disposed a search coil and a pot core adapted to receive the same and said pot core is surrounded by a circumferential wall of the insert member which bears on an internal shoulder, a bulkhead or the like of the casing sleeve disposed at an axial distance from the front end of the casing sleeve, which is closed by the closing member, with two O-rings, which are disposed in recesses in serial configuration at an axial distance from each other between the circumferential wall of the insert member and the casing sleeve.
An electric proximity sensor of this kind is already available on the market. These switches are normally used for detecting the limiting positions of pistons in hydraulic units and to this end are inserted into the cylinder wall of the appropriate hydraulic cylinder. The proximity sensors must therefore be sealed tight with respect to hydraulic fluid over rather wide and extreme temperature ranges (for example between -30.degree. C. and +100.degree. C.) a feature which cannot simply be achieved by pouring casting resin into the conventional casing sleeves which are adapted to receive the search coil with pot core as well as parts of the electrical circuit, because adhesion of the casting resin on metals is not particularly good and temperature expansion coefficients of metal and casting resins are too difficult to preclude detachment from the casing, quite apart from the shrinkage of casting resin which appears during polymerization. A fundamental remedy is also not possible even with the recently used quartz meal-loaded casting resins although these shrink or expand far less in the presence of temperature fluctuations than unloaded casting resins; however, in this case, the bond to the metallic casing sleeve is not improved and furthermore the bond between the casting resin and the quartz meal particles is loosened in the presence of temperature fluctuations and hydraulic fluid under pressure penetrates even into the smallest interface gaps.
In the known proximity sensor of the kind described hereinbefore, a cup-shaped plastics component forms the front closing member as well as the insert member and the pot core, adapted to receive the search coil, is retained by a casting resin fill against the inside of the aforementioned plastics member. This known proximity sensor suffers from two important disadvantages: on the one hand, the cup-shaped plastics closure member and insert is retained in the casing sleeve only by casting resin which penetrates from the rear to the inner O-ring and this is insufficient because of inadequate adhesion of the casting resin on the metallic casing sleeve. On the other hand, the casting resin fill, which supports the fracture-sensitive ferritic pot core must be introduced into the space behind the pot core through a port in a bulkhead which supports the plastics closure member and insert and is disposed in the casing sleeve and for this reason complete filling of the space behind the pot core and therefore the means for providing uniform mechanical support for the said pot core cannot be ensured. This reason alone is sufficient to lead to the risk of fracture of the pot core if the proximity sensor is stressed with hydraulic fluid. Since the ferritic material of the pot core has a much lower temperature expansion coefficient than casting resin which is loaded with quartz meal, it follows that the means for providing mechanical support for the pot core by means of the casting resin fill is even less satisfactory at low temperatures. Finally, there is a risk with the known proximity sensor of damage to the O-rings during assembly of the sensor: This is because the O-rings must be oversized in order to provide a seal in the axial direction between the casing sleeve and the plastics closure member and insert; when the said plastics member is inserted into the casing sleeve, the O-rings are twisted and squeezed so that after installation they are frequently severely damaged and cannot ensure proper sealing.
It was therefore the object of the invention to develop an electric proximity sensor, resistant to pressure fluid, in which the different temperature expansion coefficients of casting resin and other materials used, such as metal and ferrite, as well as the slight adhesion of the casting resin to metal do not have the detrimental consequences as in the previously described proximity sensor. Proceeding from a sensor of the kind described hereinbefore, this problem is solved in accordance with the invention in that the closing member is constructed as a component which is separate of the insert and is positively retained by securing means which are provided on the casing sleeve and is thrust in the axial direction against the outer O-ring, that the insert is provided with a bottom and is constructed substantially in cup-shape and that the space between the closing member and the bottom of the insert is filled, at least almost completely, by the pot core with search coil. In the construction according to the invention, the pot core therefore does not bear via a casting resin fill on an internal shoulder, bulkhead or the like of the casing sleeve but via the bottom of the cup-shaped insert, furthermore, the outer O-ring need not be forced when the sensor is assembled because it performs its sealing function by virtue of axial thrust applied to it by the closing member and finally, closing member, insert, pot core and search coil are positively retained in the casing sleeve. While the casting resin in the known proximity sensor must provide mechanical support, retention, sealing action, electrical insulation and possibly must also dissipate heat, most of these functions in the proximity sensor according to the invention are performed by metal parts and hydraulic seals (O-rings) while the casting resin only has a very slight stabilizing action and merely fills the small interstices between the coil windings and other gaps between the closing member, search coil, pot core, insert and casing sleeve.
Securing means provided on the casing sleeve can comprise a union nut. In a preferred embodiment of the proximity sensor according to the invention one end of the casing sleeve is flanged on to the closing member and it is advisable to provide the casing sleeve at this end with an internal shoulder on which the closing member bears. A closing member in the form of a disc represents the simplest solution but conveniently the edge zone on the outside of such a disc should have a step for the flange so that the disc cannot bulge during the flanging operation. Flanging simultaneously retains the closing member positively and therefore reliably and applies axial thrust onto the outer O-ring so that this bears sealingly all round with reliability.
The pot core must be electrically insulated with respect to the metallic casing sleeve. In known proximity switches this function was performed by the casing resin in which the pot core quasi-floats. The insert of a preferred embodiment of the proximity sensor according to the invention comprises a dural alloy and the surface thereof is hard anodized so that the hard anodized film functions as an insulating film. Another advantage of this construction is due to the fact that the pot core is supported by a metallic component, namely in the insert of duralumin and is thus reliably protected against mechanical overloading.
Finally, it is advisable for the end face of the assembly comprising the pot core and the search coil, which said assembly is nearest to the closing member, to be covered with a stratum of casting resin which is flush with the end face of the insert. In this way, a plane support surface is obtained for the closing member to ensure that the pot core is always uniformly stressed by the hydraulic pressure and cannot be stressed in bending, a feature which is of great significance in view of the fracture-sensitivity of the ferritic material of the pot core.